Iterative equalization and decoding offers some significant performance advantages on multipath/fading channels. For example, an outer rate ½ K=7 convolutional code serially concatenated with a block interleaver was shown to provide significant improvements on multipath/fading High Frequency (HF) channels, where the multipath/fading channel behaved like a rate=1 non-recursive inner code. However, research in the area of serially concatenated iterative codes has shown that additional interleaver gains exist if the inner code is recursive.
It is well known that High Frequency, Very High Frequency and Ultra High Frequency (HF/VHF/UHF) tactical (land mobile) radio channels exhibit time and frequency dispersion (i.e., delay spread and Doppler spread) because of the presence of signal reflectors and scatterers in the environment and the relative motion of a transmitter and receiver. To obtain adequate interleaver gains in a serially-concatenated system, the inner code must be recursive. In single-carrier systems, this has historically meant that the modulation used over the channel is a differential modulation or Continuous Phase Modulation (CPM).
Some communications systems generate waveforms using a recursive inner rate one (1) code and differential modulation. These communications systems often use Forward Error Correcting (FEC) and interleaving to overcome the multipath and/or fading encountered in these communications channels, for example, in wireless networks, telephone lines and similar communications networks. Recent advances in demodulation techniques for typical high frequency communications systems, however, have used advances in demodulation techniques to yield improvements in system performance by using iterative equalization and decoding. This approach helps the demodulation process by feeding back decoded bits, which, in general will be of a higher quality than the on-air received bits. As a result, a Forward Error Correction (FEC) code is used to correct some of the errors caused by multipath, fading, and noises encountered on the communications channel.
Other communications systems use serially concatenated forward error correction codes, which are iteratively decoded. Instead of using a Forward Error Correction code as an inner code, a recursive modulation, such as a Continuous Phase Modulation (CPM), is used. This fulfills a key requirement of serially concatenated codes by allowing a recursive inner code to obtain interleaver gains in the iterative decoding process. As the constellation size is increased, however, and Phase Shift Key (PSK) or Quadrature Amplitude Modulation (QAM) used, differential modulation can be difficult to implement and can have large performance degradation, i.e., 16-QAM and 64-QAM.